nixos/plymouth: add showDelay option

nixos/modules/system/boot/plymouth.nix

@@ -68,7 +68,7 @@ let
 
   configFile = pkgs.writeText "plymouthd.conf" ''
     [Daemon]
-    ShowDelay=0
+    ShowDelay=${toString cfg.showDelay}
     DeviceTimeout=8
     Theme=${cfg.theme}
     ${cfg.extraConfig}
@@ -166,6 +166,15 @@ in
         '';
       };
 
+      showDelay = mkOption {
+        type = types.numbers.nonnegative;
+        default = 0;
+        example = 0.5;
+        description = ''
+          Time (in seconds) to delay the splash screen.
+        '';
+      };
+
       extraConfig = mkOption {
         type = types.lines;
         default = "";